Some of the sponsorship choices for the London 2012 Olympics have proved controversial in recent weeks, with Dow Chemicals, BP and Rio Tinto, amongst others, coming under sustained fire from environmental and human rights campaigners. Activists say the involvement of these 'unethical' companies brings the games into disrepute and undermines claims that London 2012 will be the most sustainable Olympics ever.

With its connection to the Bhopal tragedy - in which it is claimed over 3000 people died and as many as 50,000 required hospital treatment after a massive gas leak in the Indian town of Bhopal - Dow's sponsorship generated headlines internationally and led to protests and boycott calls.

BP, responsible for the Deepwater Horizon oil spill in the Gulf of Mexico and linked to controversial tar sands oil extraction in Canada, has been accused of 'greenwashing' its way into securing the prestigious title of London 2012 'Sustainability Partner'.

But what about another principal sponsor of London 2012 - Coca Cola? The soft drinks giant claims to be the longest continuous corporate partner of the Olympic Games, stating that it "shares Olympic Values which embody the discovery of one's abilities, the spirit of competition, the pursuit of excellence, a sense of fair play and the building of a better and more-peaceful world..."

Earlier this year however the company was targeted by campaigners following an investigation by the Ecologist magazine which revealed squalid living conditions and low pay for African migrants harvesting oranges in southern Italy, many of them destined for processing into juices or concentrates used in soft drinks.

Our investigation uncovered how many migrants travel from Africa in often treacherous conditions in order to seek out a better life in Europe, or secure employment to send money back to their families. Most move between Italy's major agricultural regions - Puglia, Campania, Sicily, Calabria and Basilicata - seeking piece work during the seasonal harvests of oranges, lemons, kiwis, olives, tomatoes and melons. As many as 50,000 migrants are believed to be in Italy carrying out this work.

They typically earn 25 Euros (£21) for a day's work in the orange groves. They are often recruited by gangmasters acting on behalf of farm owners. Some gangmasters charge workers for transport to and from the orange farms or make other deductions from wages paid by farmers.

In Calabria, we found that many migrants live in appalling conditions, in run down buildings or in makeshift slums on the edge of town. There's frequently no electricity or running water. In some cases there's no functioning roof.

We visited slums where conditions were so bad that medical charities compared them to refugee camps in conflict zones. At one, in Rosarno, we reported:

It is perhaps the worst address in Western Europe. A ramshackle slum with a noisy road on one side, a railway on another, and a stagnant-looking river flowing close-by. The camp itself consists of little more than a collection of shoddily-erected canvas tents and some abandoned buildings and sheds.

Our findings prompted activists to step up their calls on multinational food and drink firms purchasing orange ingredients from the region to help address the problem. Italy's largest farmers association Coldiretti had earlier written to several companies - including Coca Cola, manufacturer of the Fanta orange drink - complaining that the prices paid for orange concentrates were unfair and in turn fostering unpleasant conditions.

Campaigners argue that the nature of the supply chain - with processors sourcing from multiple co-operatives and farmers - and the widespread use of migrant workers make it extremely difficult for companies sourcing from the region to avoid procuring 'tainted' oranges.

Coca Cola, which uses oranges from Calabria for its Fanta drinks sold in Italy, denied any wrongdoing but admitted it was unable to audit every farm or consortium whose juice may have been bought by its direct supplier.

Following widespread press coverage - the investigation also prompted street protests in Rosarno and led to Coca Cola representatives meeting with the Italian government - the drinks company said it was examining how it could help facilitate 'fair' conditions and wages for workers harvesting the oranges:

"We were concerned about the recent media allegations and are seeking to understand the reports. We are now assessing the possibility of working with our local suppliers, authorities and relevant stakeholders to carry out audits from the 'source' suppliers. We are also exploring the active role Coca-Cola can have in facilitating the discussion and standards to ensure fair conditions and pay for all workers."

The company maintained its direct Calabrian juice suppliers had been independently audited as recently as last May, and given a clean bill of health:

"Our independent audits of juice suppliers in Italy didn't raise concerns. In addition, our juice suppliers have declarations from organisations they source from stating they comply with Italian labour laws, including fair wages."

Coca Cola added that it is a "is a firm supporter of workplace and human rights", and pointed to several examples, including the convening of workshops on child labour and human trafficking.

The Calabrian orange harvest has now ended, with many migrants seeking seasonal work elsewhere - in conditions that will be disturbingly similar to those found in the orange groves. (Last year we exposed the plight of Basilicata's 'tomato slaves' with migrants toiling in the fields picking food destined for dinner plates across Europe).

Despite the pledges, campaigners say much still needs to be done to clean up Calabria's grubby orange trade. There's perhaps room for optimism however, with the recent controversy meaning - if nothing else - that the start of the next harvest should attract sufficient scrutiny to help ensure workers, and farmers, get a fair deal.

In London, as the games draw closer, the organisers are increasingly having to defend their 'ethical' credentials and choice of sponsors in the face of growing criticism. The plight of Italy's orange pickers will give them another headache - just in time for the final race to the opening ceremony.